Of course your own mileage may vary, but if you've ever had to actually pay for HDS storage (and probably the other vendors, but I've only experience with HDS), you'll know that the USP-V(M) and the VSP are all capacity based. Thankfully now the thin provisioning (Hitachi Dynamic Provisioning) comes with the VSP whereas it was an option for the USP-V(M) but the block-based tiering is still licensable. We have AMS2500s behind USP-VMs (with no disk) and it's a very expensive way to provision bulk storage i.e. 3.5" disks.
The block level tiering removes many of the requriemetns for fast disk access, so the 2.5" form factor makes more sense, since the difference between a 2.5" and a 3.5" 600 GB disk is the rotational speed and thus latency. Trust me, provisioning SATA through a USP-VM is a very expensive business. Use a mid-range array for that.